Depending on the brand of you laptop, you may have a mobile power-brick available to you. Being that its an older laptop, ebay may be a source for you.
I know the dells we used to use at work here only required the appropriate plug to go into the power brick (It was removable) to switch it from 120v to 12v. I'm pretty sure Lenovo has the same thing, we don't use them though.
Realistically you 'should avoid' an inverter on a PC/Laptop. They create a square wave instead of a proper A/C current and it can potentially damage your laptop, although I'm not sure I've ever seen one in my years as a systems analyst.